1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatuses can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning device may comprise an array of individually controllable elements, which serve to generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In lithographic projection apparatus using arrays of individually controllable elements, it is often necessary to use a plurality of such arrays in order to expose the pattern on a substrate sufficiently quickly that the though-put time for a substrate being exposed in the apparatus is economical. Furthermore, each array requires a relatively large amount of space around it for its support services such as data or control lines required for setting the pattern on each array. It is therefore not appropriate simply to illuminate all of the arrays simultaneously with a single illumination field. Illuminating all arrays with a single illumination field has the further disadvantage that a high proportion of the illumination or radiation is lost (i.e. the illumination which is transmitted between the arrays and is not received by the arrays) and might reflect off other surfaces in the lithographic apparatus, causing unwanted variations in intensity at the arrays. There may therefore be included a system that distributes radiation to a plurality of arrays from a plurality of radiation sources and/or via a plurality of radiation distribution channels.
Clearly, the radiation which passes through the radiation distribution channels can remain as uniform as possible. The prior art describes a system which is adapted to monitor continuously the radiation at the source by using a detector which is placed behind a 99% mirror at the output of a radiation source, the 99% of the radiation beam being directed to the patterning device and the remaining 1% being directed to the detector which measures the radiation intensity. This detector effectively measures the intensity of the radiation as it goes into the patterning device but not the intensity that is actually projected onto the substrate. To this end, the prior art also describes a static detector that is placed alongside the substrate. The static detector is moved into the radiation beam at set intervals in between exposures of substrates. The problem with this is that this system presumes that the transmission of the optical path between the 99% mirror and the substrate does not change between the times during which the beam intensity is sensed by the detector. Furthermore, in the case where the beam is divided into a plurality of radiation distribution channels, or in the case where there are a plurality of radiation sources, there has to be a detector for each channel and these detectors all have to be calibrated and their outputs compared to ensure that all the channels are distributing radiation of the same (or at least correct) intensity.
A further problem with the prior art is that if a variation in intensity is sensed, it is presumed to be at the radiation source that the variation occurs and the radiation source is adjusted accordingly. The detectors do not take into account variations in intensity being caused by defective individually controllable elements in the patterning device.